The invention relates generally to fiber optic delay lines, and, specifically to high speed pulse train generation using such fiber optic delay lines.
It is useful to be able to generate trains of short laser pulses which are spaced by very short time intervals. These pulse trains can be used to define observation windows for physical phenomenon which have very short durations. These short pulses, spaced together in pulse trains, can also be used to transfer data at a very high data rate.
A variety of techniques are presently known for generation of trains of short laser pulses. However, the repetition period of the pulse train, i.e., the spacing between the pulses in the pulse train, is generally large compare to the duration of the pulse itself in these prior art techniques. For example, when well known laser mode locking techniques are used, the maximum repetition frequency is usually limited to a small integer multiple of the inverse round trip transit time for the light in the resonant cavity of the laser. Therefore, the size of the resonant cavity limits the closeness of the pulse spacing and therefore the pulse spacing of the pulse train so generated. Even the smallest semi-conductor lasers can not produce pulse trains having arbitrarily close spacing between the pulses such as would be desirable for some applications.
It is desirable then to have a high speed pulse train generator which can convert a single input pulse into a train of arbitrarily closely spaced output pulses or which can compress an input pulse train into a series of output pulse trains having arbitrarily closely spaced pulses within each train. The present invention is the first demonstration of a simple apparatus and technique for generating high speed pulse trains having arbitrarily closely spaced pulses using single mode fiber optic recirculating delay lines. Such recirculating delay lines have been described in several commonly assigned, co-pending patent applications, namely: Splice-Free Fiber Optic Recirculating Memory, Ser. No. 314,473, now U.S. Pat. No. 4,473,270, filed Oct. 23, 1981; Tapped Optical Fiber Delay Line, Ser. No. 323,038, now U.S. Pat. No. 4,558,920, filed Nov. 19, 1981; and Fiber Optic Switch and Discretely Variable Delay Line, Ser. No. PCT-US82-01608, filed Nov. 12, 1982.